Alpha-amino acids are useful starting materials in the synthesis of peptides, as well as non-peptidal, pharmaceutically active peptidomimetic agents. In order to enable the synthesis of a large number of compounds from an amino acid precursor, it is advantageous to have naturally occurring and non-naturally occurring amino acids. Non-naturally occurring amino acids typically differ from natural amino acids by their stereochemistry (e.g., enantiomers), by the addition of alkyl groups or other functionalities, or both. At this time, the enantiomers of naturally occurring amino acids are much more expensive than the naturally occurring amino acids. In addition, there are only a limited number of commercially available amino acids that are functionalized or alkylated at the alpha-carbon, and often syntheses involve the use of pyrophoric or otherwise hazardous reagents. Moreover, the syntheses are often difficult to scale up to a commercially useful quantity. Consequently, there is a need for new methodologies of producing such non-naturally occurring amino acids.
Non-naturally occurring amino acids of interest include the (R)- and (S)-isomers of 2-methylcysteine, which are used in the design of pharmaceutically active moieties. Several natural products derived from these isomers have been discovered in the past few years. These natural products include desferrithiocin, from Streptomyces antibioticus; as well as tantazole A, mirabazole C, and thiangazole, all from blue-green algae. These compounds have diverse biological activities ranging from iron chelation to murine solid tumor-selective cytotoxicity to inhibition of HIV-1 infection.
Desferrithiocin, deferiprone, and related compounds represent an advance in iron chelation therapy for subjects suffering from iron overload diseases. Present therapeutic agents such as desferroxamine require parenteral administration and have a very short half-life in the body, so that patient compliance and treatment cost are serious problems for subjects receiving long-term chelation therapy. Desferrithiocin and related compounds are effective when orally administered, thereby reducing patient compliance issues. Unfortunately, (S)-2-methylcysteine, which is a precursor to the more active forms of desferrithiocin and related compounds, remains a synthetic challenge. Therefore, there is a need for novel methods of producing 2-methylcysteine at a reasonable cost, and means of isolating the desired enantiomer.
A useful and efficient method of preparing a 2-alkylcysteine involves condensing cysteine with an aryl nitrile to form a 2-arylthiazoline-4-carboxylic acid, forming a 2-arylthiazoline-4-carboxamide using an amine group comprising at least one substituted or unsubstituted alkyl group that comprises one or more chiral carbon atoms, and alkylating at the 4-position of the thiazoline ring to form a 2-aryl-4-alkyl-thiazoline-4-carboxamide. The thiazoline amide has chiral templates, which can provide face selectivity and consequently desired stereochemistry, during the delivery of an alkyl group to the 4-position of the thiazoline ring. The chiral template present in the thiazoline amide preferably produces an enantiomeric excess of one isomer.
In one embodiment, the present invention relates to a method of preparing a 2-alkylated cysteine represented by Structural Formula (I): 
or a salt thereof, wherein R1 is a substituted or unsubstituted alkyl group, the method comprising:
(a) coupling a compound (which may be an (R) or (S)-isomer or a mixture thereof) represented by Structural Formula (II): 
xe2x80x83with a substituted or unsubstituted aryl nitrile of the formula Arxe2x80x94CN, wherein Ar is a substituted or unsubstituted aryl group; thereby forming a substituted thiazoline carboxylic acid represented by Structural Formula (III): 
(b) reacting the substituted thiazoline carboxylic acid with an amine represented by Structual Formula (IV): 
xe2x80x83wherein R* is a substituted or unsubstituted alkyl group comprising one or more chiral carbon atoms and R2 is a substituted or unsubstituted alkyl or aryl group (optionally with one or more chiral carbons); thereby forming a substituted thiazoline amide represented by Structural Formula (V): 
(c) alkylating the substituted thiazoline amide with one or more bases and R1X, wherein X is a leaving group and R1 is as defined above; thereby forming an alkylated substituted thiazoline amide represented by Structural Formula (VI): 
(d) hydrolyzing the alkylated substituted thiazoline amide, thereby forming an alkylated substituted thiazoline carboxylic acid or a salt thereof, the anion of which is represented by Structural Formula (VII): 
(e) reacting the alkylated substituted thiazoline carboxylic acid with acid (preferably an inorganic acid such as HCl, HBr or sulfuric acid), thereby forming the 2-alkylated cysteine represented by Structural Formula (I).
The methods described above may additionally comprise the step of purifying or ultrapurifying the alkylated substituted thiazoline carboxylic acid or the alkylated substituted thiazoline amide. Purifying or ultrapurifying the acid or ester can comprise further resolving the enantiomers or diasteromers of the alkylated substituted thiazoline carboxylic acid or the alkylated substituted thiazoline amide. Alternatively, the 2-alkylated cysteine itself can be resolved. Additionally, the methods can comprise the isolation of the enantiomers of the synthesis products. Preferably, the (S)-enantiomer of 2-alkylcysteine is isolated, for example, (S)-2-methylcysteine.
In another aspect, the present invention relates to a method of preparing a compound represented by Structural Formula (VIII): 
or a salt thereof, the method comprising:
(a) coupling a compound (which may be an (R) or (S)-isomer or a mixture thereof) represented by Structural Formula (IX): 
xe2x80x83with a substituted of unsubstituted aryl nitrile of the formula Arxe2x80x94CN, wherein Ar is a substituted or unsubstituted aryl group; thereby forming a substituted thiazoline carboxylic acid represented by Structural Formula (X): 
(b) reacting the substituted thiazoline carboxylic acid with an amine represented by Structual Formula (XI): 
xe2x80x83wherein R* is a substituted or unsubstituted alkyl group comprising one or more chiral carbon atoms and R2 is a substituted or unsubstituted alkyl or aryl group; thereby forming a substituted thiazoline amide represented by Structural Formula (XII): 
(c) alkylating the substituted thiazoline amide with one or more bases and CH3X, wherein X is a leaving group; thereby forming an alkylated substituted thiazoline amide represented by Structural Formula (XIII): 
(d) hydrolyzing the alkylated substituted thiazoline amide, thereby forming an alkylated substituted thiazoline carboxylic acid or a salt thereof, the anion of which is represented by Structural Formula (XIV): 
(e) optionally, purifying the (S)-isomer of the alkylated substituted thiazoline carboxylic acid;
(f) reacting the (S)-isomer of the alkylated substituted thiazoline carboxylic acid with acid, thereby forming (S)-2-methylcysteine; and
(g) coupling (S)-2-methylcysteine with 2,4-dihydroxybenzonitrile, thereby forming the compound represented by Structural Formula (VIII).
Advantages of the present invention include the facile synthesis of a 2-alkyl cysteine from cysteine, an inexpensive and readily available starting material. 2-Methylcysteine prepared by the method of the present invention can be coupled to 2,4-dihydroxybenzonitrile to form 4xe2x80x2-hydroxydesazadesferrithiocin, also referred to as 4,5-dihydro-2-(2,4-dihydroxyphenyl)-4-methylthiazole-4(S)-carboxylic acid, an iron chelating agent.